As instructional designer and corporate trainer when I create P.Point presentations for classroom delivery, I usually estimate approximately 1 minute per slide. However, from practical experience I've found that the situation is different with eLearning. An excellent article I've read on the topic, mentions some good points to take into consideration (some of which you also cover here), such as complexity of subject matter, type of screen (multimedia or text), degree of user interaction per screen, time allowed for critical thinking, whether content can be skipped or not, among others. I provide the link as a reference to those interested:
http://elearningindustry.com/5-factors-to-consider-when-calculating-seat-time-in-elearning.
I definitely agree with you that the narrative length may not tell the full story as, in my opinion, what eLearning is all about is allowing users to complete the activities at their own pace. This is something many times we forget when we design eLearning courses. The fact that users have gone through the eContent once doesn't mean that they have also mastered the information presented.
As there is no clear answer to this issue, I find extremely useful your suggestion of doing live testing and calculating the average completion time.
Great post. Thanks for sharing!!